For transporting natural gas, petroleum or the like, pipelines are often laid under the ground nearby the ground surface. Construction of such pipelines is comprised of a trench digging step for digging a trench in a planned construction site; a pipeline laying step for laying a pipeline in the trench formed in the trench digging step; and a backfill step for filling earth and sand back into the trench in which the pipeline is laid. In the pipeline laying step, the following operations are repeatedly carried out: (a) short pipe transporting operation: short pipes stacked on a material handling vehicle standing by in a working site are moved to a place near the trench and arranged in a line; (b) short pipe joining operation: an adequate number of short pipes, which have been aligned in the place near the trench by the short pipe transporting operation, are joined by welding into a long pipe; and (c) long pipe joining operation: the long pipe prepared by the short pipe joining operation is joined by welding to the pipeline under construction which has been laid in the previous pipeline laying step.
Usually, a pipelayer is mainly used in first and second centering operations. The first centering operation is performed in (a) the short pipe transporting operation and (b) the short pipe joining operation in such a way that the short pipes aligned in the place near the trench are lifted to a level suited for carrying out welding operation and the central axes of the adjacent short pipes are made to coincide with each other. The second centering operation is performed in (c) the long pipe joining operation in such a way that the end of the long pipe to be joined and the end of the laid pipeline to be joined are lifted to a position suited for welding operation and the central axes of these ends are made to coincide with each other. The pipelayer is composed of a crawler-type tractor; a boom supported by the chassis frame of the tractor so as to be freely raised and lowered; a pulley block attached to the distal end of the boom; a hoist mounted on the tractor; and a hoist hook suspended from a wire rope that runs from the hoist so as to be wound over the pulley block. A hoisted load (a short pipe, long pipe or the like) hung by the hoist hook with the help of a binding tool can be raised and lowered by winding and unwinding the wire rope with the hoist.
Examples of the conventional pipelayers include mobile cranes which are self-propelled cranes such as crawler cranes, wheel cranes and track cranes. Any of the mobile cranes is constituted by an undercarriage; an upper revolving superstructure that can freely turn around upon the undercarriage; and a working implement including a boom supported by the upper revolving superstructure so as to be freely raised and lowered. Regarding the working implement, the structure of the boom itself may be of the telescopic type or lattice type, whereas the peripheral of the boom has basically the same structure. In the track crane disclosed in Japanese Patent Kokoku Publication No. 60-13960 for example, the peripheral of the boom is designed as shown in FIG. 11 in which a wire rope 102 extending from a hoist (not shown) mounted on an upper revolving superstructure 101 is wound over a top sheave 104 rotatively supported by the distal end of a boom 103 and over a hook sheave 106 provided for a hook block 105 such that load hoisting operation for raising and lowering a hoisted load W along a radial direction of the top sheave 104 can be smoothly carried out.
This conventional pipelayer has poor working efficiency because the above-described short pipe transporting operation (a) has to be entirely carried out by self-propulsion. The mobile cranes represented by the track crane 100 of Japanese Patent Kokoku Publication No. 60-13960 have a fear that when load hoisting operation including the first and second centering operations is carried out on a slope, an angle of deviation might be caused between the top sheave 104 and the wire rope 102 wound around the top sheave 104 so that the wire rope 102 deflects from the top sheave 104, resulting in malfunction.
The invention is directed to overcoming the foregoing drawbacks and a primary object of the invention is therefore to provide a pipelayer capable of transporting pipes or the like with high efficiency and stably carrying out load hoisting operation on a slope.